Traumatic brain injury (TBI) in athletes is a major and increasingly recognized public health problem. The presumed pathologic basis of the injury is multifocal microstructural traumatic axonal injury (TAI), which can be detected in vivo using diffusion tensor MRI (DTI). These structural changes have both short-term and long-term cognitive consequences. Our preliminary studies show that amateur soccer players do head the ball frequently, up to > 5,000 per year, and those who head more than 1,000- 1,500 times per year are at increased risk for structural (TAI) and functional (poorer cognitive performance) changes. Heading exposes the brain to repeated low-level (subconcussive) trauma. It is known that multiple head injuries in close succession are associated with worse outcomes. Additional work is needed to define exposure-response relationships for soccer heading, the time course for the onset and recovery from these structural and functional effects and particularly vulnerable populations. Herein, we use 3 integrated study designs to determine the influence of heading on brain structure and function. In a cross-sectional study (Aim 1), we assess the relationship of heading over the previous year to structural (TAI) and functional (cognition and post-concussion symptoms (PCS)) measures in a large sample (n=400) of amateur soccer players. To assess causality, we propose a longitudinal study to investigate the incremental effects of heading on changes in TAI, cognition and PCS over 2 years of follow-up (Aim 2). To assess the short-term relationship between heading and day-to-day variability in cognitive function and PCS as well as the predictive value of intensive measurement, we will assess heading, cognition and PCS on a daily basis for 14 days every 3 months using a hand-held device in a substudy of 100 players. We will also assess a number of potential modifiers of the association between heading and brain structure and function (e.g., Apolipoprotein E4 genotype). Specific Aims: Aim I (Cross-sectional): Determine the relationship of heading exposure to MRI evidence of traumatic axonal injury (TAI), cognitive function and post concussion symptoms (PCS). Aim II (Longitudinal): Determine the longitudinal relationships between heading and incremental changes in TAI, cognitive function, and PCS in a prospective study. Aim III (Intensive measurement): Determine the short-term effects on and variability in day-to-day cognitive function and PCS using an intensive measurement design.